High efficiency solar cell and method for manufacturing high efficiency solar cell

ABSTRACT

A solar cell including a semiconductor substrate having a first conductivity type an emitter region, having a second conductivity type opposite to the first conductivity type, on a first main surface of the semiconductor substrate an emitter electrode which is in contact with the emitter region a base region having the first conductivity type a base electrode which is in contact with the base region and an insulator film for preventing an electrical short-circuit between the emitter region and the base region, wherein the insulator film is made of a polyimide, and the insulator film has a C 6 H 11 O 2  detection count number of 100 or less when the insulator film is irradiated with Bi 5   ++  ions with an acceleration voltage of 30 kV and an ion current of 0.2 pA by a TOF-SIMS method. There can be provided a solar cell having excellent weather resistance and high photoelectric conversion characteristics.

TECHNICAL FIELD

The present invention relates to a high efficiency solar cell and amethod for manufacturing a high efficiency solar cell.

BACKGROUND ART

In recent years, a so-called back contact solar cell, in which noelectrodes are provided on a light-receiving surface to eliminateoptical loss caused by the shadow of electrodes, has been widely studiedas a technique of improving the photoelectric conversion efficiency of acrystal silicon solar cell.

FIG. 11 is a schematic view showing an example of the back surface ofthe back contact solar cell, and FIG. 12 shows a part of a cross sectiontaken along an alternate long and short dash line A in FIG. 11. As shownin FIG. 11, in a solar cell 1100, an emitter region (an emitter layer)1112 is formed on the back surface (a first main surface) of asemiconductor substrate (e.g., a crystal silicon substrate) 1110.Further, base regions (base layers) 1113 are formed in a stripe patternto sandwich the emitter region 1112 therebetween, emitter electrodes1122 are formed on the emitter region 1112, and further a plurality ofemitter electrodes 1122 are coupled through emitter bus bars (emitterbus bar electrodes) 1132. Further, base electrodes 1123 are formed onthe base regions 1113, and a plurality of base electrodes 1123 arecoupled through base bus bars (base bus bar electrodes) 1133. On theother hand, the base electrodes 1123 are electrically insulated from theemitter region 1112 through insulator films (insulator layers) 1118 andthe emitter electrodes 1122 are electrically insulated from the baseregions 1113 through the same. Furthermore, as shown in FIG. 12, thesolar cell 1100 includes passivation films 1119 on the first mainsurface and a second main surface of the semiconductor substrate 1110.It is to be noted that the passivation film 1119 is omitted in FIG. 11.

A material used for the insulator films must have characteristics suchas chemical stableness, high usable temperatures, or easiness of patternformation. A polyimide resin has been often used for the insulator filmdue to such demands (e.g., Patent Literature 1).

The insulator film of the solar cell is formed by applying an insulationprecursor (an insulator film precursor) to the substrate by screenprinting, inkjet printing, or a dispensing method. As a precursor incase of applying a polyimide resin for formation, a solution containinga polyamic acid is generally used. In this case, the polyamic acid isthermally treated to advance dehydration reaction and imidization,whereby the polyimide resin is provided.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Unexamined Patent

SUMMARY OF INVENTION Technical Problem

However, when a solar cell using the polyimide resin derived from theprecursor containing a polyamic acid is put in a hot and humid state,there often occurs a problem that electrical contact between thesubstrate and electrodes in contact with the insulator is degraded.

The present invention has been made in view of the above problems, andan object thereof is to provide a solar cell which has excellent weatherresistance and high photoelectric conversion characteristics and amethod for manufacturing such a solar cell.

Solution to Problem

To achieve the object, the present invention provides a solar cellcomprising: a semiconductor substrate having a first conductivity type;an emitter region, having a second conductivity type opposite to thefirst conductivity type, on a first main surface of the semiconductorsubstrate; an emitter electrode which is in contact with the emitterregion; a base region having the first conductivity type; a baseelectrode which is in contact with the base region; and an insulatorfilm for preventing an electrical short-circuit between the emitterregion and the base region, wherein

the insulator film is made of a polyimide, and

the insulator film has a C₆H₁₁O₂ detection count number of 100 or lesswhen the insulator film is irradiated with Bi₅ ⁺⁺ ions at anacceleration voltage of 30 kV and an ion current of 0.2 pA by a TOF-SIMSmethod.

Such a solar cell is superior in weather resistance and has highphotoelectric conversion characteristics.

Additionally, it is preferable that the insulator film is formed toelectrically insulate the emitter region from the base electrode.

Such a solar cell can avoid the electrical short-circuit of the emitterregion and the base electrode by using the insulator film.

Additionally, it is preferable that the insulator film is formed toelectrically insulate the base region from the emitter electrode.

Such a solar cell can avoid the electrical short-circuit of the baseregion and the emitter electrode by using the insulator film.

Additionally, it is preferable that the insulator film is formed toelectrically insulate the emitter electrode from the base electrode.

Such a solar cell can avoid the electrical short-circuit of the emitterelectrode and the base electrode by using the insulator film.

Additionally, it is preferable that the semiconductor substrate is acrystal silicon substrate.

When the semiconductor substrate is the crystal silicon substrate, thesolar cell which has good power generation efficiency can be provided atlow cost.

Furthermore, the present invention provides a photovoltaic moduleincluding solar cells according to the present invention electricallyconnected to each other.

The solar cells according to the present invention can be electricallyconnected to provide the photovoltaic module.

Furthermore, the present invention provides a photovoltaic powergeneration system including a plurality of photovoltaic modulesaccording to the present invention connected to each other.

The plurality of photovoltaic modules each of which is constituted byelectrically connecting the solar cells according to the presentinvention can be connected to provide the photovoltaic power generationsystem.

Furthermore, the present invention provides a method for manufacturing asolar cell comprising the steps of:

forming, on a first main surface of a semiconductor substrate having afirst conductivity type, an emitter region having a second conductivitytype opposite to the first conductivity type, and a base region havingthe first conductivity type;

forming an emitter electrode which is in contact with the emitter regionand a base electrode which is in contact with the base region; andforming a polyimide containing no carboxy group as an insulator film forpreventing an electrical short-circuit between the emitter region andthe base region.

According to such a method, it is possible to manufacture the solar cellwhich has excellent weather resistance and high photoelectric conversioncharacteristics.

Additionally, it is preferable to use a crystal silicon substrate as thesemiconductor substrate.

The method according to the present invention is particularly preferablefor manufacture of the solar cell including the crystal siliconsubstrate.

Advantageous Effects of Invention

The solar cell of the present invention is superior in weatherresistance and has high photoelectric conversion characteristics.According to the method of the present invention, it is possible toprovide a back contact solar cell which has high efficiency andexcellent weather resistance without requiring a process change.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view showing a backside structure of a back contact solarcell according to the present invention;

FIG. 2 is a view showing a cross-sectional structure of the back contactsolar cell according to the present invention;

FIG. 3 is a graph showing ion intensity spectrums in TOF-SIMS analysisof an insulator film;

FIG. 4 is a view showing a backside structure of a back contact solarcell according to another embodiment of the present invention;

FIG. 5 is a view showing a cross-sectional structure of the back contactsolar cell according to another embodiment of the present invention;

FIG. 6 is a view showing a cross-sectional structure of the back contactsolar cell at a different position according to another embodiment ofthe present invention;

FIG. 7 is a view showing a backside structure of a back contact solarcell of still another embodiment according to the present invention;

FIG. 8 is a view showing a cross-sectional structure of the back contactsolar cell of still another embodiment according to the presentinvention;

FIG. 9 is a view showing a photovoltaic module according to the presentinvention;

FIG. 10 is a view showing a photovoltaic power generation systemaccording to the present invention;

FIG. 11 is a view showing a backside structure of a general back contactsolar cell; and

FIG. 12 is a view showing a cross-sectional structure of the generalback contact solar cell.

DESCRIPTION OF EMBODIMENTS

The present invention will now be described in detail hereinafter.

As described above, a solar cell which is superior in weather resistanceand has high photoelectric conversion characteristics has been demanded.

The present inventors have conducted the earnest examinations to achievethe object. Consequently, they have found that the problem can be solvedby a solar cell including: a semiconductor substrate having a firstconductivity type; an emitter region, having a second conductivity typeopposite to the first conductivity type, on a first main surface of thesemiconductor substrate; an emitter electrode which is in contact withthe emitter region; a base region having the first conductivity type; abase electrode which is in contact with the base region; and aninsulator film for preventing an electrical short-circuit between theemitter region and the base region, wherein

the insulator film is made of a polyimide, and

the insulator film has a C₆H₁₁O₂ detection count number of 100 or lesswhen the insulator film is irradiated with Bi₅ ⁺⁺ ions at anacceleration voltage of 30 kV and an ion current of 0.2 pA by a TOF-SIMSmethod, thereby bringing the present invention to completion.

As described above, a method for manufacturing a solar cell which issuperior in weather resistance and has high photoelectric conversioncharacteristics has been demanded.

The present inventors have conducted the earnest examinations to achievethe object. Consequently, they have found that the problem can be solvedby a method for manufacturing a solar cell including the steps of:

forming, on a first main surface of a semiconductor substrate having afirst conductivity type, an emitter region having a second conductivitytype opposite to the first conductivity type, and a base region havingthe first conductivity type;

forming an emitter electrode which is in contact with the emitter regionand a base electrode which is in contact with the base region; and

forming a polyimide containing no carboxy group as an insulator film forpreventing an electrical short-circuit between the emitter region andthe base region, thereby bringing the present invention to completion.

Hereinafter, embodiments of the present invention will now bespecifically described with reference to the drawings, but the presentinvention is not restricted thereto.

[Solar Cell]

Although a solar cell according to the present invention will now bedescribed hereinafter with reference to the drawings, the presentinvention is not restricted thereto. FIG. 1 is a view showing a backsidestructure of a back contact solar cell according to the presentinvention. Further, FIG. 2 is a view showing a cross-sectional structureof the back contact solar cell according to the present invention, andshows a part of a cross section taken along an alternate long and shortdash line A in FIG. 1. As shown in FIGS. 1 and 2, a solar cell 100according to the present invention includes a semiconductor substrate110 having a first conductivity type. Furthermore, on a first mainsurface of the semiconductor substrate 110 are provided an emitterregion 112 having a second conductivity type opposite to the firstconductivity type, emitter electrodes 122 which are in contact with theemitter region 112, base regions 113 having the first conductivity type,base electrodes 123 which are in contact with the base regions 113, andinsulator films 118 which prevent an electrical short-circuit betweenthe emitter region 112 and the base regions 113.

Moreover, as shown in FIG. 1, the solar cell 100 according to thepresent invention usually includes base bus bars 133 configured tofurther collect currents which can be provided from the base electrodes123. Additionally, it usually includes emitter bus bars 132 configuredto further collect currents which can be provided from the emitterelectrodes 122. Further, as shown in FIG. 2, passivation films 119 areusually provided on the first main surface and a second main surface ofthe semiconductor substrate 110. It is to be noted that the passivationfilm 119 is omitted in FIG. 1.

In the solar cell according to the present invention, the insulator film118 is made of a polyimide and has a C₆H₁₁O₂ detection count number of100 or less when the insulator film 118 is irradiated with ions(divalent ions of a bismuth pentamer) at an acceleration voltage of 30kV and an ion current of 0.2 pA (picoampere) by a time-of-flightsecondary ion mass spectrometry (TOF-SIMS) method. This detection countnumber is approximately a detection lower limit of the TOF-SIMS.Further, an organic substance containing a carboxy group can be detectedas, e.g., a peak of C₆H₁₁O₂ whose m/z value (m: an ion mass number, z:an ion charge number) appears in the vicinity of 115 in secondary ionsejected by Bi ion irradiation in the TOS-SIMS. Thus, it can be saidthat, in the solar cell according to the present invention, eachinsulator film is made of a polyimide which hardly contains carboxygroups or does not contain the same at all. When an organic substancecontaining carboxy groups remains in the insulator film, i.e., when thecount number exceeds 100, weather resistance is considerably lowered.Although reasons for this are yet to be clear, it can be considered thata carboxylic acid is generated from the carboxy group derived from anamic acid with moisture absorption of the insulator film and acts onelectrodes themselves or an interface between the electrodes and siliconto degrade electric resistance.

Although the shape of the insulator film is not restricted inparticular, it may be, e.g., a rectangular shape. In this case, thelength of one side of the insulator film may be, e.g., 0.01 mm to 50 mm.Further, the thickness of the insulator film may be, e.g., 1 to 60 μm.Adopting such length and thickness enables further improving insulationproperties. Furthermore, since the insulator films are not excessivelyformed, it is possible to assuredly manufacture a desired solar cell,namely, a solar cell in which each insulator film is made of a polyimidecontaining no carboxy group.

Moreover, as shown in FIG. 1, it is preferable to form the solar cell100 according to the present invention so that the insulator films 118electrically insulate the emitter region 112 from the base electrodes123. Such a solar cell enables avoiding an electrical short-circuitbetween the emitter region and the base electrodes by using theinsulator films.

Additionally, as shown in FIG. 1, it is preferable to form the solarcell 100 according to the present invention so that the insulator films118 electrically insulate the base regions 113 from the emitterelectrodes 122. Such a solar cell enables avoiding an electricalshort-circuit between the base regions and the emitter electrodes.

Further, as shown in FIG. 1, it is preferable to form the solar cell 100according to the present invention so that the insulator films 118electrically insulate the emitter electrodes 122 from the baseelectrodes 123. Such a solar cell enables avoiding an electricalshort-circuit between the emitter electrodes and the base electrodes.

Furthermore, it is preferable for the semiconductor substrate 110 to bea crystal silicon substrate. When the semiconductor substrate is thecrystal silicon substrate in this manner, the solar cell with the goodpower generation efficiency can be provided at low costs.

[Method for Manufacturing Solar Cell]

Although the method according to the present invention can be applied togeneral solar cells using insulators, an example of the method will nowbe described. For example, the method according to the present inventioncan be applied to the solar cell shown in FIGS. 1 and 2. Hereinafter, aspecific method for manufacturing a solar cell will be described withreference to FIGS. 1 and 2 in conjunction with a case using an N-typesubstrate.

First, an N-type semiconductor substrate such as an N-type crystalsilicon substrate is prepared. Specifically, high-purity silicon may bedoped with a pentad such as phosphorous, arsenic, or antimony to preparean as-cut single crystal {100} N-type silicon substrate having aspecific resistance of 0.1 to 5 Ω·cm.

Next, small irregularities called a texture may be formed on a lightreceiving surface of the semiconductor substrate to reduce reflectanceof the solar cell.

Then, as shown in FIGS. 1 and 2, an emitter region 112 having a secondconductivity type opposite to that of the semiconductor substrate 110and base regions 113 having a first conductivity type equal to that ofthe semiconductor substrate 110 are formed on the back surface (a firstmain surface) of the semiconductor substrate 110. A method for formingthe emitter region 112 and the base regions 113 is not restricted inparticular, and a well-known method can be used. For example, theemitter region 112 can be formed by vapor phase diffusion using BBr₃ orthe like. The base regions 113 can be formed by vapor phase diffusionusing phosphorous oxychloride. Furthermore, when the emitter region 112and the base regions 113 are formed, a diffusion mask composed of asilicon oxide film, a silicon nitride film, or the like may be used toform the emitter region 112 and the base regions 113 with desiredshapes. For example, as shown in FIG. 1, stripe-shaped base regions 113may be formed, while emitter region 112 are formed except for areaswhere the base regions 113 are formed.

Then, a passivation film 119 constituted of a silicon nitride film, asilicon oxide film, or the like is formed on each of the light receivingsurface and the back surface of the semiconductor substrate 110. Thesilicon nitride film can be formed by a CVD method, and the siliconoxide film can be formed by the CVD method or a thermal oxidationmethod.

Then, emitter electrodes 122 which are in contact with the emitterregion 112, and base electrodes 123 which are in contact with the baseregions 113 are formed. In case of the solar cell having the backsidestructure shown in FIG. 1, the emitter electrodes 122 and the baseelectrodes 123 which extend in a horizontal direction are formed on theemitter region 112 and the base regions 113.

Although a method for forming the electrodes is not restricted inparticular, they could be formed by screen printing or dispenserformation using a conductive paste in terms of productivity. In thiscase, the emitter electrodes 122 and the base electrodes 123 are formedas follows: an Ag paste obtained by mixing Ag powder and glass frit withan organic binder is applied to the emitter region 112 and the baseregions 113 with the passivation film 119 being inserted; the Ag pasteis then dried; and the Ag paste is fired at about 700 to 880° C. for 1to 30 minutes. As a result of this heat treatment, the passivation film119 is eroded by the Ag paste, whereby the electrodes make electricalcontact with the silicon.

Furthermore, plating may be applied. In this case, since the substratesurface is necessarily exposed at positions where the electrodes are tobe formed, the passivation film 119 at such positions is removed by,e.g., laser ablation.

Then, a polyimide containing no carboxy group is formed as the insulatorfilms 118 for preventing an electrical short-circuit between the emitterregion 112 and the base regions 113. In case of the solar cell havingthe backside structure shown in FIG. 1, the insulator films 118 areformed at intersecting points of the emitter region 112 and base busbars 133 and intersecting points of the base regions 113 and emitter busbars 132.

Although a method for forming the insulator films is not restricted inparticular, the film is preferably formed by applying a paste of aninsulation precursor by screen printing, inkjet printing, or dispensedcoating in terms of productivity. In this case, although a processdiffers to some extent depending on the insulation precursor to be used,it is often the case that the precursor is printed and thereafter driedin the air at approximately 70° C. to 150° C. for approximately oneminute to 10 minutes, and then an actual curing is performed. Althoughan actual curing method also varies depending on the insulationprecursor, it may be a thermosetting type, a UV (ultraviolet) curingtype, or the like. As heat treatment conditions for performing theactual curing, for example, heat treatment can be carried out in the airat 200° C. to 400° C. for approximately 10 seconds to 15 minutes.However, each insulator film 118 is made of a polyimide containing nocarboxy group.

Here, an organic substance containing carboxy groups can be detected as,e.g., a peak of C₆H₁₁O₂ whose m/z value (m: an ion mass number, z: anion charge number) appears in the vicinity of 115 in secondary ionsejected by Bi ion irradiation in the TOS-SIMS. Thus, in the methodaccording to the present invention, an actually cured insulator filmhaving a C₆H₁₁O₂ detection count number of 100 or less when it isirradiated with Bi₅ ⁺⁺ ions at an acceleration voltage of 30 kV and anion current of 0.2 pA by the TOF-SIMS method can be regarded as theinsulator film made of a polyimide containing no carboxy group. It is tobe noted that the detection count number (100) is roughly a detectionlower limit of the TOF-SIMS method.

When the organic substance containing carboxy groups remains in theinsulator film, the weather resistance is considerably lowered. Althoughreasons for this are yet to be clear, it can be considered that acarboxylic acid is generated from the carboxy group derived from an amicacid with moisture absorption of the insulator film and acts onelectrodes themselves or an interface between the electrodes and siliconto degrade electric resistance.

As described above, an amic acid solution is dehydrated/imidized by heattreatment, but imidization is partially incomplete due to a coating filmthickness, heat treatment conditions, or the like, and the precursorremains in some situations. In such a case, there is a possibility thatoptimizing the coating film thickness and the heat treatment conditionscan avoid the problem.

However, it is preferable to use an insulator film precursor containingno amic acid. In this case, for example, a soluble polyimide coatingagent using a soluble polyimide described in Japanese Unexamined PatentApplication Publication No. 2015-180721 can be adopted.

The soluble polyimide will now be described hereinafter. The solublepolyimide is, e.g., a polyimide powder provided by a solid phasepolymerization method, and this polyimide can be the polyimide powderwhich is soluble in an amide solvent and whose weight-average molecularweight based on GPC (gel permeation chromatography) is 10000 or more.

Illustrative examples of the amide solvent include N-methyl-2-pyrolidone(NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), andthe like.

Furthermore, the polyimide powder can be manufactured bysolid-phase-polymerizing a salt consisting of a tetracarboxylic acid ora tetracarboxylic diester and diamine in the presence of a solvent in anamount of 1 mass % or more and 30 mass % or less with respect to a massof this salt.

Here, the solid phase polymerization method is a method for advancingpolymerization reaction in a solid state.

Here, the tetracarboxylic diester means a tetracarboxylic acid dimethylester, a tetracarboxylic acid diethyl ester, a tetracarboxylicdiisopropyl acid ester, or the like.

Preferred examples of the tetracarboxylic acid includecyclohexane-1,2,4,5-tetracarboxylic acid (H-PMA), pyromellitic acid(PMA), 3,3′,4,4′-biphenyltetracarboxylic acid (BPA),2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane (6-FA), diesters of thesesubstances, and the like.

Preferred examples of the diamine include 4,4′-diaminodiphenyl ether(DADE), 2,2-bis[4-(4-aminophenyxy)phenyl]propane (BAPP),1,3-bis(aminomethyl) cyclohexane (AMC), isophoronediamine (IPDA),2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (6F-BAPP),polypropylene glycol bis(2-aminoethyl) ether (PGAE), and the like.

The solvent used in the solid phase polymerization is a solvent whichcan dissolve the polyimide powder to be generated by the solid phasepolymerization. Illustrative examples thereof include general solventssuch as an amide solvent, an ether solvent, a water ester solvent, aketone solvent, and the like; and the amide solvent is preferred. As theamide solvent, NMP, DMF, DMAc, or the like described above can be used.

Next, an insulator film made of the soluble polyimide and an amic acidtype insulator film will be described with reference to FIG. 3.

FIG. 3 shows spectrums in the vicinity of m/z=115 provided as a resultof irradiating four types of cured insulator films A, B, C, and D withBi₅ ⁺⁺ ions at an acceleration voltage of 30 kV and an ion current of0.2 pA by using the TOF-SIMS (TOF-SIMS300 manufactured by ION-TOF). Ofthese films, the sample A is an amic acid type polyimide film that iscured by, after coating and drying at 90° C. for five minutes, heattreatment at 250° C. for 10 minutes. In this sample, a clear peak ofC₆H₁₁O₂ can be observed at m/z=115.08, which indicates that the film isincompletely imidized. On the other hand, the samples B to D arepolyimide films made of soluble polyimides having differentcompositions, but a peak derived from the amic acid, which can be seenin the sample A, cannot be observed in these samples. Table 1 shows thedetails of the insulator films A, B, C, and D.

TABLE 1 Type of C₆H₁₁O₂ Composition of insulator Heat insulatordetection film precursor (volume %) Insulator Film treatment film countResin film thickness conditions precursor number component SolventOthers A about drying at Amic acid about 1600 30 67 3 20 μm 90° C. for 5solution B minutes, Soluble 100 or 25 66 9 C curing at polyimide less 2669 5 D 250° C. for 10 coating 30 65 5 minutes agent

As shown in Table 1, when the soluble polyimide coating agent is used asthe insulator film precursor containing no amic acid, a polyimidecontaining no carboxy group can be easily formed as the insulator film118.

After forming the insulator film 118, the emitter bus bars 132 and thebase bus bars 133 are formed. A thermosetting conductive paste which iscurable at room temperature to 350° C. or a UV curable conductive pastecan be used for these bus bars, and such a paste could be applied byscreen printing or dispenser formation. As shown in FIG. 1, theinsulator films 118 can electrically insulate the emitter electrodes 122from the base bus bars 133 and also electrically insulate the baseelectrodes 123 from the emitter bus bars 132, while the emitterelectrodes 122 and the emitter bus bars 132 can be configured to beelectrically continuous, and the base electrodes 123 and the base busbars 133 can be configured to be electrically continuous.

According to the method of the present invention, it is possible toprovide a back contact solar cell having excellent durability and highefficiency. Although the foregoing examples are in conjunction with thecase where the substrate is an N-type substrate, the method of thepresent invention can also be applied to a case where the substrate is aP-type substrate. That is, an N-type layer may be provided as an emitterlayer while a P-type layer may be provided as a base layer.

The present invention can be also applied to solar cells shown in FIGS.4 to 8. FIG. 4 is a view showing a back surface of anemitter-wrap-through solar cell 400 as a solar cell according to anotherembodiment of the present invention, and each of FIG. 5 (across-sectional view taken along an alternate long and short dash lineA) and FIG. 6 (a cross-sectional view taken along an alternate long andshort dash line B) shows a part of a cross section taken along thealternate long and short dash line A or B in FIG. 4. In FIGS. 5 and 6,the light receiving surface faces downward.

In this embodiment, most part of the back surface of a substrate 110 isoccupied by a base region 113 and base electrodes 123 formed on the baseregion 113, and emitter regions 112 are formed into island shapes inregions sandwiched between insulator films 118. On the other hand, alight receiving surface is occupied by an emitter region 112 thatcommunicates with the emitter regions 112 on the back surface throughvia holes opened in the substrate 110. Moreover, emitter electrodes 122are connected between the light-receiving surface and the back surfacethrough the via holes. A passivation film 119 is formed on the lightreceiving surface. Furthermore, base bus bars 133 are linearly formed onthe base electrode 123 on the back surface of the substrate 110.Moreover, emitter bus bars 132 are linearly formed on the emitterregions 112 and the emitter electrodes 122 on the back surface of thesubstrate 110. However, as shown in FIG. 6, in each region where theemitter bus bar 132 crosses the base electrode 123, the base electrode123 is covered with the insulator film 118.

FIG. 7 is a view showing the back surface of a solar cell 700 of stillanother embodiment according to the present invention, and FIG. 8 is across-sectional view showing a part of a cross section taken along analternate long and short dash line A in FIG. 7. In this embodiment, baseregions 113 on the back surface of a substrate 110 are formed in such amanner that they are surrounded by an emitter region 112, and each baseelectrode 123 is in contact with the base region 113 and insulated froma passivation film 119 through an insulator film 118 in a portion wherethe base electrode 123 overlaps the emitter region 112. It is to benoted that both a passivation effect and insulation performance cannotbe generally achieved in the passivation film 119 in most cases, andhence it is preferable to assuredly achieve insulation by using aninsulator. The passivation film 119 is omitted in FIG. 7. Additionally,at an outer peripheral portion of the back surface of the substrate 110,an emitter bus bar 132 which is coupled with a plurality of emitterelectrodes 122 is formed, and a base bus bar 133 which is coupled with aplurality of base electrodes 123 is formed.

Further, FIG. 7 shows that each base region 113 is linearly formed belowthe base electrode 123, but the present invention is not restrictedthereto, each base region 113 may be formed into a dot shape and theseregions may be linearly aligned. In this case, the base electrodes 123are formed in such a manner that they are insulated from the emitterregion 112 through the insulator films 118 and connect the base regions113.

[Photovoltaic Module]

A photovoltaic module according to the present invention is constitutedby electrically connecting the solar cells according to the presentinvention. For example, when a plurality of solar cells as providedabove are electrically connected in series, a photovoltaic module isprovided. FIG. 9 shows an example of a photovoltaic module 916. Apositive electrode 923 of a solar cell is electrically connected to anegative electrode 922 of an adjacent solar cell through a tab 912, andthe number of solar cells (solar battery cells) 900 required for apredetermined output are connected. Although not shown, the connectedsolar cells 900 are sealed with a cover glass, a filter, and further aback sheet. As the cover glass, a soda-lime glass is extensively used.Further, as the filler, ethylene vinyl acetate, polyolefin, silicone, orthe like is used. As the back sheet, a functional film usingpolyethylene terephthalate is generally adopted. In addition, thepositive electrode 923 of one solar cell is connected with a positiveelectrode terminal 913 of the photovoltaic module 916, and the negativeelectrode 922 of another solar cell is connected with a negativeelectrode terminal 914 of the photovoltaic module 916.

[Photovoltaic Power Generation System]

A photovoltaic power generation system according to the presentinvention is constituted by connecting a plurality of photovoltaicmodules according to the present invention. FIG. 10 shows a basicstructure of a photovoltaic power generation system provided by couplingthe modules of the present invention with each other. As shown in FIG.10, a plurality of photovoltaic modules 1016 are coupled in seriesthrough wiring 1015 and supply generated power to an external loadcircuit 1018 via an inverter 1017. Although not shown in this drawing,this system may further include a secondary battery which stores thegenerated power.

EXAMPLES

Although the present invention will now be more specifically describedhereinafter with reference to examples and a comparative example, thepresent invention is not restricted to the following examples.

Example 1

A solar cell shown in FIGS. 1 and 2 was manufactured by using the methodaccording to the present invention.

Using a phosphorous-doped <100> n-type as-cut silicon substrate having asize of 150 mm square, a thickness of 200 μm, and a specific resistanceof 1 Ω·cm, an emitter region and base regions were formed on the backsurface of the substrate as shown in FIG. 1.

This substrate was thermally treated in an oxygen atmosphere at 900° C.for 10 minutes, and oxide silicon films were formed on both surfaces ofthe substrate. Subsequently, silicon nitride films with a film thicknessof 90 nm were further formed on both the surfaces of the substrate byplasma CVD.

Then, an Ag paste was applied to the emitter region and the base regionsby screen printing, and heat treatment was performed at 800° C. forthree seconds to cure the Ag paste, thereby forming emitter electrodesand base electrodes.

Then, an insulator film B in FIG. 3 was formed on a part of the emitterregion and a part of the base electrode by screen printing. It is to benoted that conditions and others for forming the insulator film B are asdescribed in Table 1.

Subsequently, to form emitter bus bars and base bus bars, athermosetting Ag paste was applied by screen printing, and a heattreatment was performed at 200° C. for five minutes to cure the paste,thereby providing a solar cell.

Conductive wires are soldered to the bus bars in the solar cell andsealed with a white glass plate, a silicon resin, and a back sheet tofabricate a single cell module.

Initial output characteristics of the manufactured single cell modulewere measured by using pseudo sunlight of a xenon lamp light sourcetype, then the module was stored in a temperature and humidity testingchamber set to 85° C. and 85% relative humidity for 2000 hours, andthereafter the measurement was again performed.

Example 2

In the same solar cell manufacturing process using the same substrate asthat in Example 1, as an insulator film which is formed on a part of theemitter region and a part of the base electrode, an insulator film C inFIG. 3 was used to manufacture a solar cell. It is to be noted thatconditions and others for forming the insulator film C are as describedin Table 1.

Then, a single cell module was manufactured in the same manner as inExample 1.

Initial output characteristics of the manufactured single cell modulewere measured by using pseudo sunlight of a xenon lamp light sourcetype, then the module was stored in a temperature and humidity testingchamber set to 85° C. and 85% relative humidity for 2000 hours, andthereafter the measurement was again performed.

Comparative Example 1

In the same solar cell manufacturing process using the same substrate asthat in Example 1, as an insulator film which is formed on a part of theemitter region and a part of the base electrode, an insulator film A inFIG. 3 was applied to manufacture a solar cell. It is to be noted thatconditions and others for forming the insulator film A are as describedin Table 1.

Then, a single cell module was manufactured in the same manner as inExample 1.

Initial output characteristics of the manufactured single cell modulewere measured by using pseudo sunlight of a xenon lamp light sourcetype, then the module was stored in a temperature and humidity testingchamber set to 85° C. and 85% relative humidity for 2000 hours, andthereafter the measurement was again performed.

Table 2 shows the solar cell initial characteristics and characteristicdecreasing rate after elapse of 2000 hours of the high temperature andhigh humidity test according to Examples 1 and 2 and Comparative Example1 described above.

TABLE 2 Comparative Solar cell characteristics Example 1 Example 2Example 1 Short- Initial stage 39.4 39.5 39.4 circuit After 2000 39.439.4 38.2 current hours (0.0) (0.3) (3.0) [mA/cm²] (decreasing rate %)Open Initial stage 0.680 0.679 0.679 circuit After 2000 0.680 0.6800.679 voltage hours (0.0) (−0.1) (0.0) [V] (decreasing rate %) FillInitial stage 80.9 80.7 81.0 factor After 2000 80.5 80.5 72.0 [%] hours(0.5) (0.2) (11.1) (decreasing rate %) Conversion Initial stage 21.721.6 21.7 efficiency After 2000 21.6 21.6 18.7 [%] hours (0.5) (0.4)(13.8) (decreasing rate %)

As shown in Table 2, the initial characteristics of each of Example 1and Example 2 (examples where the polyimide containing no carboxy groupwas formed as the insulator film) were comparable to those ofComparative Example 1 (an example where the polyimide containing carboxygroups was formed as the insulator film), but had greatly improveddurability. This result shows that the solar cell with high efficiencyand high weather resistance can be achieved by the present inventionwithout requiring any additional step.

It is to be noted that the present invention is not restricted to theforegoing embodiment. The foregoing embodiment is an illustrativeexample, and any example which has substantially the same configurationand exerts the same functions and effects as the technical conceptdescribed in claims of the present invention is included in thetechnical scope of the present invention.

The invention claimed is:
 1. A method for manufacturing a solar cell comprising the steps of: forming, on a first main surface of a semiconductor substrate having a first conductivity type, an emitter region having a second conductivity type opposite to the first conductivity type, and a base region having the first conductivity type; forming an emitter electrode which is in contact with the emitter region and a base electrode which is in contact with the base region; and forming a polyimide containing no carboxy group, as an insulator film, from an insulator film precursor containing no amic acid, wherein the insulator film is configured to prevent an electrical short-circuit between the emitter region and the base region.
 2. The method for manufacturing a solar cell according to claim 1, wherein a crystal silicon substrate is used as the semiconductor substrate. 